1. Field of the Invention
This invention relates to an interferometer apparatus for measuring the position, the momentum, the shape or the like of an object by the use of a coherent light beam, and particularly is suitable for a laser interferometer apparatus widely used for highly accurate position measurement.
2. Related Background Art
For example, in an exposing apparatus for the manufacture of semiconductors, a laser interferometer apparatus is used as a measuring apparatus for measuring the coordinates of a wafer stage or mask stage for highly accurately positioning a wafer. The laser interferometer apparatus is an apparatus for applying a laser beam to two members displaced relative to each other, making a signal obtained by photoelectrically converting the interference beam of the reflected two laser beams, for example, into a pulse signal and increment or decrement counting this pulse signal to thereby measure the relative displacement of the two members. There is known an apparatus in which temperature-controlled air is blown to stabilize the fluctuation of the air in the optical path of the laser beam (for example, U.S. Pat. No. 4,814,625).
In a laser interferometer apparatus of this kind, except when it is used in vacuum, the accuracy of the correction of any variation in the length of a measuring optical path (so-called wavelength correction) resulting from any variation in the refractive index of the atmospheric gas through which a laser beam passes greatly affects the final measurement accuracy. The prior-art apparatus of this kind has adopted one of two methods, i.e., a method of measuring and correcting temperature, atmospheric pressure, humidity, etc. (which are the factors of a variation in refractive index) by suitable sensors, and a method of installing a refractometer utilizing an interferometer and directly measuring and correcting the refractive index of the atmospheric gas (for example, U.S. Pat. No. 5,141,318 and U.S. Pat. No. 4,215,938).
However, the former method of estimating the refractive index from temperature, etc., for example, the method using a temperature sensor, has suffered from the inconvenience that it cannot grasp the average temperature variation of the entire measuring optical path and it is low in response speed and cannot properly measure a fast variation and thus, is insufficient in accuracy. Also, in the latter method using a refractometer, the area for measuring the refractive index differs from the area of the optical path which is the actual object of the measurement by the laser beam, and this has led to the inconvenience that accuracy is limited by the local irregularity of the refractive index.